¹ Marine Chemistry Department, National Institute of Oceanography and Fisheries, Elanfoshy, Alexandri, Egypt

² Chemistry Department, Faculty of Science, Alexandria University, Ibrahimia, P.O. Box 426, Alexandria 21321, Egypt

Abstract

The inhibition of steel corrosion in hydrochloric acid and sodium chloride solutions by Nicotiana leaves extract was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Polarization curves showed that the extract behaves as a mixed type inhibitor in acidic medium, while it acts as anodic type in neutral medium. Impedance measurements indicated that the size of the semicircles obtained increased with increasing the concentration of the extract and decreasing the chloride ion concentration. Nicotiana leaves extract was found to be more effective in controlling corrosion of steel in acidic solution than in neutral one. The activation parameters of the corrosion reaction of steel in acid media in absence and presence of nicotiana leaves extract were also calculated and discussed.

Introduction

Inhibitors are generally used to reduce corrosion of metallic material in different media. Several chemical compounds have been investigated as corrosion inhibitors for mild steel in acid and neutral media [1-7]. The use of chemical inhibitors has been limited because of the environmental threat. Recently, due to environmental regulations, plant extracts have again become important because they are environmentally acceptable, readily available and a renewable source for a wide range of needed inhibitors. Plant extracts are viewed as an incredibly rich source of naturally synthesized chemical compounds that can be extracted by simple procedures with low cost. As new trend, several researchers used natural products as corrosion inhibitors. Abdel-Gaber et al. [8] investigated the effect of extracts of some plants on the corrosion of steel in aqueous 1 M sulphuric acid by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. Oguzie [9] studied also the corrosion inhibition of mild steel in 2 M hydrochloric acid and 1 M sulphuric acid by extracts of selected plants using a gasometric technique at temperatures of 30 and 60 °C. The studied plant materials include leaf extracts of Occimum viridis, Telferia occidentalis, Azadirachta indica and Hibiscus sabdariffa, as well as extracts from the seeds of Garcinia kola. The effect of lupine and damsissa extracts on the corrosion of steel in 0.5 M Na₂SO₄ solution free from and containing 0.01 or 0.1 M NaCl were examined by potentiodynamic and electrochemical impedance spectroscopy techniques [10]. Also, olive leaves extract was studied as scale and corrosion inhibitor for steel surface in brine solution [11]. Earlier et al. [12] showed that the actual inhibitors in the plant extracts are usually alkaloids and other organic nitrogen bases, as well as carbohydrates, proteins and their acid hydrolysis products.

It also contains many amino acids (glutamic acid and aspartic acid) and organic acids (malice acid)[13]. Additionally, another study indicates that 7-dehydrocholesterol is found in Nicotiana glauca [14].

This work aims to investigate Nicotiana glauca extract as corrosion inhibitor of mild steel in acidic and neutral solutions.

Electrochemical tests

Electrochemical impedance and polarization curve measurements were achieved using a Gill AC instrument. The frequency range for EIS measurements was 0.1 ≤ f ≤ 1 × 10³ Hz with applied potential signal amplitude of 10 mV around the rest potential. The data were obtained in a three-electrode mode; platinum sheet and saturated calomel electrodes were used as counter and reference electrodes respectively. The material used for constructing the working electrode was steel with the following chemical composition (wt.%): C, 0.21; S, 0.04; Mn, 2.5; P, 0.04; Si, 0.35; balance Fe. The steel was encapsulated in epoxy resin in such a way that only one surface was left uncovered. The exposed area (1 cm²) was mechanically abraded with a series of emery papers of variable grades, starting with a coarse one and proceeding in steps to the finest (800) grade. The samples were then washed thoroughly with double distilled water, followed with A.R. ethanol and finally with distilled water, just before insertion in the cell. Before polarization and EIS measurements, the working electrode was introduced into the test solution and left for 10 min at the open circuit potential. Polarization curve measurements were obtained at a scan rate of 20 mV/min starting from cathodic potential (Ecorr -250 mV) going to anodic direction. To test the reliability and reproducibility of the measurements, duplicate experiments were performed in each case at the same conditions.

Solutions preparation

Double distilled water and analytical reagent-grade NaCl and HCl were used for preparing the solutions. Stock solution of the plant extracts was obtained by drying the plant for 1 h in an oven at 70 °C and grinding to powdery form. A 10 g sample of the powder was refluxed in 100 mL double distilled water for 1 h. The refluxed solution was filtered to remove any contamination. The concentration of the stock solution was determining by evaporating 10 mL of the filtrate and weighing the residue.

Results and discussion

Acidic solution

Potentiodynamic polarization results

The displayed data show that increasing plant extract concentration decreases the corrosion current density (i_corr) and shifts the corrosion potential (E_corr) to more noble values. The percentage of inhibition efficiency (%P) was calculated from polarization measurements using the relation

where i_o and i are the corrosion current density in the absence and presence of plant extract, respectively. The slight variations in anodic and cathodic Tafel slopes (βa and βc) indicate that the inhibiting action is taking place by the simple blocking of the available cathodic and anodic sites on the metal surface.

Electrochemical impedance spectroscopy results

These plots indicate that the dissolution process occurs under activation control. The impedance response consisted of characteristic semicircles. These semicircles are of a capacitive type whose size increases with increasing Nicotiana leaves extract concentration.

The data indicate that the increasing of the charge transfer resistance is associated with a decrease in the double layer capacitance. The decrease in the C_dl values could be attributed to the adsorption of the chemical constituents of Nicotiana leaves extract at the metal surface. It has been reported that the adsorption process on the metal surface is characterized by a decrease in C_dl [15].

The % P were calculated from impedance measurements using the relation

where R_ct0 and R_ct are the charge transfer resistances in the absence and presence of the plant extract.

where Ea is the apparent effective activation energy, T the absolute temperature, R the universal gas constant and A the Arrhenius pre-exponential factor. An alternative formulation of the Arrhenius equation is the transition state equation:

where N is the Avogadro's number, h the Plank's constant, ΔH* the enthalpy of activation and ΔS* is the entropy of activation.

The higher values of Ea and ΔH* in presence of Nicotiana extract than those obtained for pure 1 M HCl indicated that, when the temperature was raised, a decrease in protection efficiency was obtained [18]. On the other hand, the slight decrease in their values in presence of the extract in 0.1 M HCl solution indicates that the effectiveness of this extract at higher temperature is not affected [19].The negative value of ΔS* implies that the activated complex represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex [20,21]. It is reported that the rate-determining step for the hydrogen evolution reaction on iron is the recombination of the adsorbed hydrogen atom to form H2 molecules [22-24].

Therefore, the transition state of the rate-determining recombination step represents a more orderly arrangement relative to the initial state, and, hence, a negative value of the entropy of activation is obtained in absence or presence of the extract. The value of ΔS* is nearly the same in presence of the extract in 1 M HCl, while higher negative value of ΔS* is obtained in 0.1 M HCl, which indicates that more ordered recombination step is obtained in presence of the extract molecules in low acid concentration rather than in high concentration.

Neutral medium

Potentiodynamic polarization results

The data clarify that the addition of plant extract decreases the corrosion current density (icorr) and shifts the corrosion potential (Ecorr) to less negative values, especially in 0.1 M NaCl. The values of anodic and cathodic Tafel slopes (βa and βc) are slightly changed with the addition of Nicotiana extract, indicating that the inhibition process occurs without changing the mechanism [11]. As in acid medium, the inhibition efficiency of Nicotiana increases in 0.1 M solution more than in 1 M due to the effect of chloride ion concentration.

Electrochemical impedance spectroscopy results

The impedance, Z, of CPE is presented by

The data show that the charge transfer resistance increases by increasing the concentration of the extract, which indicates that the extract acts as a corrosion inhibitor for steel in 0.1 M NaCl rather than in 1 M solution. Moreover, increasing R_f and R_ct is accompanied by decreasing in the none ideal double layer capacitance values, Q_dl, suggesting that inhibition process takes place through adsorption of the chemical constituents of Nicotiana extracts on the metallic surface.

The polarization and impedance measurements indicated that the extract inhibits the corrosion processes by blocking the available cathodic and/or anodic sites of the metal surface through adsorption of the extract chemical constituents on the metal/solution interface. This phenomenon could take place via [26]: (i) electrostatic attraction between the positively charged protonated nitrogen atom and negatively charged mild steel surface (cathodic sites), (ii) dipole-type interaction between unshared electron pairs of oxygen atom or π electrons interaction with the vacant, low energy d-orbitals of Fe surface atoms (anodic sites), and (iii) a combination of all of the above (mixed type). The lower inhibition efficiency of Nicotiana extract in high chloride concentration solutions can be explained on the basis that increasing the concentration of chloride ion hinders the adsorption of extract molecules on the metal surface.

Conclusions

The corrosion of mild steel in HCl and NaCl solutions is significantly reduced upon the addition of Nicotiana leaves extract. The inhibition efficiency increased with increasing the concentration of the extract and decreasing chloride ion concentration.

The results of potentiodynamic polarization measurements demonstrate that the Nicotiana leaves extract behaves as a mixed type inhibitor in acidic medium, while it acts as anodic type in neutral medium. The extract was found to be more effective in controlling corrosion of steel in acid solution than in neutral one. At low acid concentration, protection efficiency of Nicotiana extract is not affected by raising the temperature and extract molecules increase the ordering of recombination step. On the other hand, in high acid concentration, the efficiency of the extract decreases with raising the temperature and the recombination step ordering is not affected.

References